A water absorbent resin has been hitherto used as a component for hygienic materials such as sanitary cotton, disposable diaper, and absorbents for other kinds of body fluid. As typical examples of the water absorbent resin, hydrolyzate of starch-acrylonitrile graft polymer, neutralized starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic ester copolymer, hydrolyzate of acrylonitrile copolymer or acrylamide copolymer, and cross-linked product thereof, and partially neutralized cross-linked acrylic acid may be cited. These water absorbent resins have an internal cross-linked structure and are in-soluble in water. The characteristics of such a water absorbent resin include for example high absorption capacity, high absorption speed, high gel strength, and fully satisfactory suction power necessary for sucking water from a medium. The water absorbing properties are affected by cross-link density, and an increase in the cross-link density typically leads to an increase in the gel strength but a decrease in the amount of water absorbed. Particularly, increased absorption capacity typically leads to reduced absorption speed, reduced gel strength, and reduced suction power, for example. The water absorbent resin having improved absorption capacity, therefore, would possibly induce inhomogeneous absorption of water and lead to aggregation of absorbent particles when the water absorbent resin particles contact with water, and also induce dramatic decrease in absorption speed because the water is not diffused throughout the entire volumes of water absorbent resin particles.
For obtaining a water absorbent resin having high absorption capacity and a comparatively satisfactory absorption speed, a method for coating a surface of water absorbent resin particles with a surfactant or a nonvolatile hydrocarbon has been available. This method indeed can exalt the dispersibility of initially absorbed water but does not have sufficient effects on enhancing absorption speed and suction power of individual resin particles.
As a means to produce a polyacrylic acid based water absorbent polymer having improved water absorbing properties, a method which comprises heating an aqueous composition of a polymer having a partial alkali metal salt of polyacrylic acid as a main component and having a low cross-link density in the presence of a water-soluble peroxide radical initiating agent thereby introducing a cross-link therein by radical cross-linking has been proposed (U.S. Pat. No. 4,910,250). It is difficult to distribute uniformly internal cross-links in the polymer and uneasy to adjust the cross-link density. Accordingly, a polymer which contains water-soluble polyacrylic acid gel having low cross-link density is obtained and then the polymer is heated together with a persulfate added thereto as a polymerization initiator. U.S. Pat. No. 4,910,250 states that excellent water absorbing properties can be attained and a water absorbent resin having no stickiness can be obtained because the adjustment of the amount of the initiator to be added can allow precise control of cross-link density and uniform presence of cross-link in the polymer.
While the persulfate which is used in the U.S. Pat. No. 4,910,250 is decomposed by heat, it is also decomposed by ultraviolet rays to generate radicals (J. Phys. Chem., 1975, 79, 2693, J. Photochem. Photobiol., A. 1988, 44, 243). Since the persulfate acts as a polymerization initiator, the aqueous solution of a water-soluble vinyl monomer, when exposed to radiation, undergoes polymerization and radical cross-linking simultaneously (JP-A 2004-99,789). A reaction system has also been known, which comprises adding a hydrophilic polymer component and a photo-polymerization initiator, further adding a cross-linking agent thereto, and irradiating them with ultraviolet rays to form an internal cross-link (WO 2004/031253).
Further, a method which comprises treating a surface of a water absorbent resin to increase cross-link density of the surface of water absorbent resin has also been known (U.S. Pat. Nos. 4,666,983 and 5,422,405, for example). Such water absorbent resins as cited in the preceding publications comprise a reactive functional group on their surfaces. By adding a surface cross-linking agent capable of reacting with the functional groups in order to introduce cross-links between the functional groups, cross-link density on the surface of water absorbent resin can be increased and a water absorbent resin having excellent water absorbing properties even under pressure can be obtained.
Further, since the use of the surface cross-linking agent requires a high temperature and a long time for the reaction of forming cross-link and entails the problem of suffering persistence of unaltered cross-linking agent, a method which comprises contacting an aqueous solution containing a peroxide radical initiating agent with a resin, heating the resin to decompose the radical initiating agent and introduce cross-links into polymer molecular chains in the neighborhood of the surface of the resin has been proposed (U.S. Pat. No. 4,783,510). In the working example, a water absorbent resin exhibiting exalted absorption capacity was obtained by heating with superheated steam at 130° C. for 6 minutes.